Isoimmunization of marmosets with platelets from another species of marmoset leads to marked thrombocytopenia of the recipient concomitant with the development of antibodies towards donor platelet antigens. The induction of this hematologic disease is achieved by intramuscular immunization of the animals with platelets suspended in complete Freund's adjuvant. Current studies are being conducted to investigate other immunization routes which will predictably yield acute or chronic thrombocytopenia. Another primary objective is to test the thesis that destruction of platelets in vivo may also be a cellular-mediated phenomenon. This is being pursued through in vitro tests in which we are attempting to stimulate autologous lymphocytes with platelets coated with antibodies. The role of the spleen in the disease process is also to be tested through utilization of spenectomized marmosets before or after the isoimmunizations. The nature of the antibody formed in these studies is being characterized by several different assay procedures, namely, its lytic activity in the presence or absence of complement, its ability to be detected by attachment to the platelet by indirect immunofluorescence and finally its ability to cause release of serontonin through injury of the platelet membrane.